This invention relates to perforating guns used in well completion operations. More particularly, the present invention relates to a single-wire selective gun perforating system capable of selecting and firing in an arbitrary order each gun in a plurality of guns connected in a firing string where the selection process includes safeguards for detecting faults in the firing string prior to the firing of a gun.
Typical prior-art perforating guns generally used in well completion operations consist of a plurality of guns connected vertically to form an assembly or firing string suitable for lowering into a well borehole. Each gun will contain one or more shaped charges. Each charge will have a detonator or blasting cap connectable to a firing wire for receiving an electrical firing pulse to detonate the charges.
It is often desirable in well completion operations to have each gun selectable for firing rather than having all guns firing at the same time. Firing all guns at the same time produces a perforation spacing determined by the spacing of the guns in the string, usually in a closely-spaced arrangement. On the other hand, individual detonation of the charges permits perforations to be made at various selected depths, and in various selected (often widely separated) zones. As each charge is detonated, the string can be repositioned to the next level where another perforation is desired, and another gun fired. This process can continue until the proper perforation spacing is obtained with the desired number of shots. A further benefit is obtained from the single detonation of the guns--verification that each gun fired and that the proper number of perforations was obtained.
However, the selection and firing of a single gun in the string may involve failures which would prevent the proper firing of the modules. A failure could occur in the gun to be selected that would prevent it from firing; a failure could occur causing the firing of a wrong gun which will be eventually detected; or a failure could occur which caused the undetected firing of a wrong gun. Any one of these failures, especially in many of the prior art devices, would defeat the purposes of having selective firing of the guns in the perforation operations.
Many selective firing systems and methods have been used in the prior art to select a gun for firing from among the plurality of guns in the string. U.S. Pat. No. 4,051,907 discloses one such system comprising a surface control unit for controlling the selection and firing of the guns in a firing string comprised of a subsurface master unit operatively connected to a plurality of identical slave sub units or firing modules that may be armed and fired in an arbitrary order under control of the master unit and an operator.
Sequencing through the firing modules for selection of a module to be fired is under control of the surface located control unit. The selection process begins at the uppermost firing module closest to the master unit. Each firing module contains a pulse counter which receives pulses from the surface via the master slave unit when that module has been connected to the firing line power. A predetermined number of pulses (8 pulses) sequences the counter through nine counts. At selected counts, certain operations are effected in the module. For example, at count 4 a current pulse is placed on the firing line, at count 5 a switch is closed to charge a firing capacitor with the voltage currently on the firing line, at count 6 a firing pulse whose amplitude is equal to the current voltage on the firing line is applied to a blocking zener diode which is connected to a firing switch (the firing switch is not closed because the voltage on the firing line is not greater than the break over voltage of the zener diode), and at count 9 a pass-through switch is closed to pass the firing line power on down to the next lower module in the string.
The above described process is then repeated for the next module to be connected to the firing line power. As long as eight pulses are issued without a change in the firing line power, the sequencing through the firing modules will continue, one at a time. When the firing module to be selected and fired is reached, only six pulses will be issued by the master unit under control of the operator. These six pulses take the pulse counter in the firing module to be selected to a count of five which closes the switch which connects the firing line to the firing capacitor. At this point, the operator at the surface activates the arm switch which raises the firing line voltage, and thus the voltage on the firing capacitor, to a value sufficient to detonate the charge when the capacitor is discharged into the blasting cap. Six pulses arm the firing module with one more pulse causing a closing of the firing switch to occur since the firing line voltage is now greater than the blocking zener diode voltage to permit the firing switch to be closed. Closure of the firing switch connects the firing capacitor across the blasting cap circuit.
These prior-art selective perforating systems, such as that disclosed in U.S. Pat. No. 4,051,907, suffer from several disadvantages. One disadvantage is the need for elaborate surface and subsurface circuitry with continuous supervision and interaction required between the surface and subsurface circuitry during the selection process to effect the selection and arming of the firing modules. Another disadvantage is that sequencing through the firing modules is solely under control of the surface equipment. Another disadvantage is the lack of any safeguards for detecting faults in the firing string which will prohibit the proper firing of a single selected module.
Accordingly, it would be advantageous to provide a single-wire selective perforating system which provides for the automatic sequencing through the firing modules in a sequence, one at a time, under control of the modules themselves until a module to be selected is receiving power from the firing line. At that time, the module can be selected. It would also be advantageous to provide a single-wire selective perforating system which includes safeguards for determining if a single module has been connected in the sequence to the firing line and is operating within predicted power limits thereby insuring that one module is being selected for firing and only that module will be fired by the firing pulse.